This invention relates to gas flow measuring devices. More specifically, this invention relates to gas flow measuring devices of the soap film type.
Soap film type gas flowmeters are known and are typically used to measure the flow rate of gases in many applications, such as gas chromatography, leak detection, laboratory analyzers, and calibration of flow devices. Generally, a soap film type gas flowmeter includes a vertically extending flow tube of known diameter with an inlet near the bottom connected to the source of gas whose flow rate is to be measured. Also at the lower end of the flow tube is an arrangement for forming a soap film or bubble which can travel upwardly through the tube. The film forming arrangement is usually manually operable, and typically comprises a squeeze bulb containing a soapy solution. The gas entering the inlet flows upwardly through the tube and carries the film upwardly therethrough. At opposite ends of a measured length of the flow tube there are mounted respective detecting devices which detect the passage of the soap film thereby, and produce an electrical signal in response thereto. These signals are coupled to a timing device which measures the time required for the film to travel from one detecting device to the other. This time measurement determines the flow rate of the gas, which is then typically displayed in an analog or digital fashion.
Known gas flowmeters of the soap film type suffer from several disadvantages. Firstly, during use the soap film solution frequently accumulates at the top of the flow tube around the outlet, and flows down the outside of the tube into the detecting region, which causes contamination of the detecting devices and causes inaccurate measurements to be obtained. In addition, the detecting devices are usually fixed at a predetermined distance from the source of the soap film in the tube. For relatively slow flow rates, the transit time of the soap film from its point of origin to the first detector may be so long that the measuring instrument is annoyingly slow. This problem is exacerbated when several measurements are to be taken to ensure that the source is adjusted to the proper flow rate.
Perhaps more importantly, however, when measuring the flow rate of relatively light gases, such as helium or hydrogen, the heavier ambient air diffuses down into the gas flow tube to the emerging soap film. This creates a pressure differential on opposite sides of the soap bubble, causing some of the lighter gas to diffuse through the soap film. This phenomenon results in flow measurement errors, which cannot readily be accounted for and which are extremely undesirable.
An additional problem with known gas flowmeters lies in the relatively coarse tolerances used to manufacture the flow tube, which is usually made from glass. In known flowmeters, the device is typically calibrated at the factory using the original tube. When the tube is later damaged or destroyed in use, the entire meter must normally be returned to the factory, so that the device can be recalibrated with a new tube. This procedure is time consuming and inconvenient.
Efforts to devise a soap film type gas flowmeter devoid of the above disadvantages have not been successful to date.